There are known various methods of growing sapphire tubes by pulling a pin-shaped seed from a molten annular film formed at an end face of a die wherein said film continuously receives a supply of melt through capillary tubes in the die contained in a crucible. Yet, among the problems encountered in growing tubes there is one of impaired mechanical strength and transparency brought about by peculiarities in the distribution of temperatures in the tube grown. The point is that elastic temperature strains coming into play in a tubular sapphire crystal give rise to dislocations, block-like structure and impair the mechanical strength of the crystal after all.
The method referred to above has been embodied in an apparatus incorporating an air-tight chamber with an induction heater for the indirect heating made in the form of a conductive cylindrical container with the working coil wound around same. Disposed in the container bore is a crucible with dies located below the upper edge of the container which is provided with horizontal heat-insulating shields arranged in the bore of said container above the crucible and pierced with perforations enabling the tube or tubes pulled to pass therethrough. Inherent in the known apparatus are the same drawbacks as present in the above method which originate from the failure to take into account the distribution of temperatures in the tube pulled. Another disadvantage calling for attention is that in the known apparatus the phenomenon of overheating of the crucible is likely to be observed with the result that gas bubbles are formed which make their way into the liquid film and thence into the crystal grown, reducing thus its mechanical strength. The effect of said disadvantages inherent in the method and apparatus known is especially felt in growing thin-walled sapphire tubes.